You need 3 points.
Probably the easiest ones would be the intercepts
(0,0,8)
(0,-4,0) and
(16,0,0)
using the first two, a direction vector would be (0,4,8)
using the last two, a direction vector would be (16,4,0)
we could shorten these to (0,1,2) and (4,1,0)

check:
a normal to our plane is (1,-4,2)
taking the dot product of that normal with each of my two vectors yields zero, as expected.